1. Field of the Invention
The present invention relates to a packet transmission control apparatus and a packet transmission control method which perform transmission control of packets to a plurality of mobile stations.
The present invention relates particularly to a packet transmission control apparatus and a packet transmission control method which perform transmission control of downlink packets in a mobile communication system.
2. Description of the Related Art
In a downlink of a mobile communication system, one physical channel can be shared among mobile stations which belong to a radio base station. Hereinafter, the physical channel used in such a case is called a “downlink shared channel”.
In this downlink shared channel, the radio base station controls the transmission order of packets to the plurality of mobile stations with which the radio base station communicates, in accordance with an instantaneous radio quality between the radio base station and each mobile station, so as to improve throughput that the radio base station can provide, in other words, a system capacity.
This control of packet transmission order by the radio base station is called “scheduling”. It is known that, by applying the scheduling to packet transmission, channel capacity increases, or communication condition improves.
Generally, it is considered that the conventional scheduling targets on packets in which requirements for transmission delays are not so strict.
Incidentally, with regard to standardization of the third generation mobile communication system, so-called IMT-2000, there are “3GPP/3GPP2 (Third-Generation Partnership Project/Third-Generation Partnership Project 2)”. Standard specifications have been developed as “W-CDMA system” in the 3GPP, and standard specifications have been developed as “cdma2000 system” in the 3GPP2.
In the 3GPP, “HSDPA (High Speed Downlink Packet Access)”, which is a high-speed packet transmission system in the downlink direction, has been standardized based upon a prospect that high-speed and high-capacity traffic will increase especially in the downlink due to downloading from databases and websites and the like, as the Internet has rapidly expanded in recent years.
Moreover, in the 3GPP2, “1x-EV DO”, which is a transmission system only for high-speed data in the downlink direction, has been standardized from the same viewpoint as above. In the “1x-EV DO” of the cdma2000 system, “DO” means “Date Only”.
For example, in the HSDPA, a scheme for controlling a modulation scheme and a coding rate of respective radio channels in accordance with the radio quality between each mobile station and a radio base station (this scheme is called, for example, AMCS (Adaptive Modulation and Coding Scheme) in the HSPDA), and the scheduling which is operated in a cycle of few milliseconds, are used in a combination. Thus, it is possible to improve throughput for individual mobile stations as well as throughput of the entire system.
“Round Robin Scheduler” is widely known as a scheduling algorithm in a radio base station. The “Round Robin Scheduler” controls the transmission order of packets waiting for transmission, by assigning the downlink shared channel sequentially to mobile stations (for example, mobile stations #1 to #2 to #3 . . . ) which belong to the radio base station.
Moreover, “Proportional Fairness Scheduler” and “Max C/I (Maximum C/I) Scheduler” are known as scheduling algorithms in a radio base station. The “Proportional Fairness Scheduler” and the “Max C/I Scheduler” control the transmission order of packets waiting for transmission based upon the instantaneous transmission rate and the average transmission rate of packets to each mobile station.
The “Proportional Fairness Scheduling” is a scheduling algorithm which assigns a transmission queue and also supports fairness amongst the mobile stations, in accordance with instantaneous changes in downlink radio qualities of the individual mobile stations.
Hereinbelow, the “Proportional Fairness Scheduling” is briefly described with reference to FIG. 1. FIG. 1 is a flowchart showing the operation of the “Proportional Fairness Scheduler” mounted on a radio base station.
In the “Proportional Fairness Scheduling”, a value of an evaluation function of each mobile station which belongs to the radio base station is calculated based upon the measured instantaneous transmission rate of packets to each mobile station (e.g. the instantaneous radio quality between each mobile station and the radio base station) and the measured average transmission rate of packets to each mobile station (e.g. the average radio quality between each mobile station and the radio base station), and thereafter, a transmission queue is assigned to a mobile station maximizing the value of the evaluation function.
As shown in FIG. 1, in step S1001, the radio base station sets initial values as follows:    n=1 (n: a subscript of a mobile station)    Cmax=0 (Cmax: a maximum value of an evaluation function Cn)    nmax=0 (nmax: a subscript of the mobile station maximizing the value Cn of the evaluation function)
In step S1002, the radio base station measures elements required in calculating the value Cn of the evaluation function, specifically, an instantaneous transmission rate Rn of packets to each mobile station, and an average transmission rate Rn of packets to each mobile station.
In step S1003, the radio base station calculates the value Cn of the evaluation function, by using the values measured in the step S1002, according to the following equation.
  Cn  =      Rn          Rn      _      
In step S1004, the radio base station determines whether or not the value Cn of the evaluation function calculated in the step S1003 exceeds a maximum value Cmax of the evaluation function.
Here, Cmax=0. Therefore, the determination in the step S1004 is YES, and in step S1005, the radio base station sets the value Cn of the evaluation function calculated in the step S1003 at the maximum value Cmax of the evaluation function, and also sets “1” at “nmax”.
Thereafter, in step S1006, the radio base station increments “n” by “+1”, and determines whether or not “n” exceeds “N (the number of mobile stations communicating with the radio base station)” in step S1007.
Where “n” does not exceed “N”, the operation repeats the steps from S1002 to S1006, thus obtaining N values of the evaluation function sequentially.
In step S1008, the radio base station selects a mobile station #nmax maximizing the value Cn of the evaluation function, and assigns a transmission queue to the mobile station #nmax.
Here, the numerator of the evaluation function is an “instantaneous transmission rate of packets to the mobile station (e.g. instantaneous radio quality between the radio station and each mobile station)”, and the denominator of the evaluation function is an “average transmission rate of packets to the mobile station (e.g. the average radio quality between the radio station and each mobile station)”. Thus, the “Proportional Fairness Scheduler” operates so that the probability of assigning transmission queue to the mobile station #n, whose instantaneous transmission rate is larger than the above average transmission rate, becomes higher.
According to the “Proportional Fairness Scheduler”, transmission queues are assigned to a mobile station which has high instantaneous transmission rate, even when the average transmission rate of the mobile station is large or small. Therefore, the “Proportional Fairness scheduler” can realize both “fairness in terms of time” and “high cell throughput by user diversity gain”.
However, the operation of the conventional “Proportional Fairness Scheduler” is based on the assumption that the capability is the same in all mobile stations. Thus, there has been a problem in that fairness in terms of time cannot always be provided by the conventional “Proportional Fairness Scheduler” in case where the capability is different in a plurality of mobile stations.
For example, a mobile station has the Receive diversity (Rx diversity) function as a receiver capability and another mobile station dose not have the Receive diversity function as a receiver capability.
In this case, the value Cn of the evaluation function will not always become large in the former mobile station, because the average information rate and the receivable number of information bits (the instantaneous information rate) increase simultaneously, with the Receive diversity function, when the transmission queue is assigned.
Thus, it is adapted to set the same average information rate to all mobile stations because of using Rn in the denominator of the evaluation function for the conventional Proportional Fairness Scheduler.
In other words, the conventional Proportional Fairness Scheduler tends to reduce the frequency of assigning transmission queues to a mobile station which has high capability. This causes that the conventional Proportional Fairness Scheduler is adapted to negate the high capability of the former mobile station.
Further, according to the conventional Proportional Fairness Scheduler, typically, the changes of radio quality becomes smaller in a mobile station which has the Receive diversity function than in a mobile station which does not have the Receive diversity function. When the evaluation function is used as an instantaneous radio quality to the average radio quality, the values Cn of the evaluation function becomes larger in little case, or the maximum value of the evaluation function becomes smaller. As a result, there has been a problem in that the number of times transmission queues are assigned is reduced.
It is explained, by referring to FIG. 2, that the radio quality changes of a mobile station having the Receive diversity function becomes smaller. As shown in FIG. 2, the radio qualities which change without correlation are combined by the mobile station which has the Receive diversity function, and therefore the radio quality changes are reduced.
In short, there has been a problem in that the conventional Proportional Fairness Scheduler is adapted to reduce the number of times transmission queues are assigned to mobile stations which have high capabilities, and to negate the effect of the high capabilities.
Further, as those mobile stations have high capability at the risk of high price, large power requirements, or large in size, it is reasonable that the number of times transmission queues are assigned should be increased. However, the conventional Proportional Fairness Scheduler is adapted to act in conflict with this concept.